Image forming apparatus generating conversion condition of measurement unit

ABSTRACT

The image forming apparatus including: a controller controlling the image forming unit to form a test image, controlling the measurement unit to measure the test image on the image bearing member, and controlling the transfer unit to transfer the test image to the sheet; a reception unit receiving a user instruction; a second generation unit generating the conversion condition based on the user instruction and the measurement result of the test image, wherein the sample image includes a first sample image, a second sample image, and a third sample image, a color difference between the first and second sample images is smaller than a threshold value, a color difference between the second and third sample images is larger than the threshold value, density of the first sample image is higher than density of the second sample image and density of the third sample image.

BACKGROUND OF THE INVENTION

Field of the Invention

The present invention relates to correction control for correcting acharacteristic of an image formed by an image forming apparatus.

Description of the Related Art

An image forming apparatus employing an electrophotographic method formsan electrostatic latent image based on image data on a photoconductorand develops the electrostatic latent image using a developing agent(toner) included in a developer device so as to form an image. Tocontrol the image formed by the image forming apparatus to have desireddensity, a measurement image formed by the image forming apparatus ismeasured and a correction condition is corrected in accordance with aresult of the measurement.

U.S. Pat. No. 8,229,307 discloses an image forming apparatus which formsa measurement image on a photoconductor, measures the measurement imageby a sensor, and corrects a correction condition in accordance with aresult of the measurement performed by the sensor.

However, even in a case where the correction condition is corrected,desired density of an image formed on a sheet may not be obtained. Thisis caused by a measurement error of the sensor. If an error occurs inthe result of the measurement performed by the sensor, density of theimage formed by the image forming apparatus may not be corrected withhigh accuracy.

SUMMARY OF THE INVENTION

An image forming apparatus of the present invention includes acorrection unit configured to correct image data based on a correctioncondition, an image forming unit configured to form an image based onthe corrected image data, an image bearing member configured to bear theimage formed by the image forming unit, a transfer unit configured totransfer the image on the image bearing member to a sheet, a measurementunit configured to measure a measurement image on the image bearingmember, a conversion unit configured to convert a measurement result ofthe measurement image by the measurement unit, based on a conversioncondition, a first generation unit configured to generate the correctioncondition based on the measurement result converted by the conversionunit, an obtaining unit configured to control the image forming unit toform a test image based on test image data, control the measurement unitto measure the test image on the image bearing member, and obtain ameasurement result of the test image by the measurement unit, acontroller configured to control the image forming unit to form a testimage based on test image data, and control the transfer unit totransfer the test image formed on the image bearing member to the sheet,a reception unit configured to receive a user instruction based on aresult of a user having compared a sample image and the test imagetransferred on the sheet, and a second generation unit configured togenerate the conversion condition based on the user instruction and themeasurement result obtained by the obtaining unit. The sample imageincludes a first sample image, a second sample image, and a third sampleimage. A color difference between the first and second sample images issmaller than a threshold value. A color difference between the secondand third sample images is larger than the threshold value. Density ofthe first sample image is higher than density of the second sampleimage. The density of the first sample image is higher than density ofthe third sample image.

Further features of the present invention will become apparent from thefollowing description of exemplary embodiments with reference to theattached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view schematically illustrating an imageforming apparatus.

FIG. 2 is a block diagram illustrating the image forming apparatus.

FIG. 3 is a diagram schematically illustrating a main portion of adensity detection sensor.

FIG. 4 is a graph illustrating a conversion table.

FIGS. 5A and 5B are graphs illustrating the relationships among a tonerapplying amount, print density, and a sensor output value.

FIGS. 6A to 6C are diagrams schematically illustrating a process ofgenerating correction data used to correct a γ LUT.

FIG. 7 is a flowchart illustrating automatic tone correction.

FIG. 8 is a diagram illustrating a pattern image.

FIG. 9 is a diagram schematically illustrating a sample chart.

FIG. 10 is a diagram schematically illustrating a test sheet.

FIG. 11 is a flowchart illustrating visual correction.

FIG. 12 is a diagram schematically illustrating a state in which theconversion table is updated.

FIG. 13 is a graph illustrating color differences between adjacentsample images in the sample chart.

FIG. 14 is a graph illustrating density of sample images in the samplechart.

FIGS. 15A and 15B are graphs illustrating accumulated color differencesbetween sample images.

DESCRIPTION OF THE EMBODIMENTS

FIG. 1 is a cross-sectional view schematically illustrating an imageforming apparatus which forms a full-color image. The image formingapparatus includes four image forming stations 10Y, 10M, 10C, and 10K,and forms an image on a sheet P. The image forming station 10Y forms ayellow image, the image forming station 10M forms a magenta image, theimage forming station 10C forms a cyan image, and the image formingstation 10K forms a black image. A full-color image is formed on anintermediate transfer belt 6 by transforming images of the individualcolor components formed by the image forming stations 10Y, 10M, 10C, and10K to the intermediate transfer belt 6 in an overlapping manner. Thefull-color image on the intermediate transfer belt 6 is transferred onthe sheet P. After a fixing unit 100 fixes the image on the sheet P, thesheet P is discharged from the image forming apparatus.

The image forming stations 10Y, 10M, 10C, and 10K have the sameconfiguration except that the image forming stations 10Y, 10M, 10C, and10K accommodate different toners of different color components.Hereinafter, a configuration of the image forming station 10Y isdescribed, and configurations of the other image forming stations 10M,10C, and 10K are omitted.

The image forming station 10Y includes a photoconductive drum 1Y havinga photoconductor formed on a surface thereof, a charger 2Y having acharging roller connected to a high voltage power source, an exposingdevice 3Y which exposes the photoconductive drum 1Y so as to form anelectrostatic latent image, and a developer device 4Y which develops theelectrostatic latent image using toner. The image forming station 10Yfurther includes a primary transfer roller 7Y which is disposed so as toface the photoconductive drum 1Y with the intermediate transfer belt 6described below therebetween, and a drum cleaner 8Y which collects toneradhering to the photoconductive drum 1Y.

The intermediate transfer belt 6 is supported by a plurality of rollersand is driven to rotate by a driving roller connected to a motor notillustrated. A pair of secondary transfer rollers 9 which sandwich theintermediate transfer belt 6 therebetween forms a secondary transfer nipportion T2. A sheet P is conveyed toward the secondary transfer nipportion T2. The pair of secondary transfer rollers 9 is connected to apower source unit (not illustrated). The power source unit applies asecondary transfer voltage to the pair of secondary transfer rollers 9while the sheet P passes the secondary transfer nip portion T2 so thatthe image on the intermediate transfer belt 6 is transferred to thesheet P in the secondary transfer nip portion T2. A belt cleaner 11presses an elastic member of a plate shape on the intermediate transferbelt 6 so as to collect toner on the intermediate transfer belt 6.Furthermore, a density detection sensor 5 measures a measurement imageformed on the intermediate transfer belt 6. The density detection sensor5 will be described later with reference to FIG. 4.

A fixing device 100 includes a pair of rollers which presses a sheet anda heater which heats a sheet. The fixing device 100 heats the sheet Pwhile pressing the sheet P so as to firmly fix the image on the sheet Pwhich has not been fixed to the sheet P. The sheet P on which the imageis fixed is output from the image forming apparatus.

Next, an image forming operation of forming an image based on image datasupplied from a PC, a scanner, or the like, not illustrated, which isperformed by the image forming apparatus will be described.

In the image forming station 10Y, the photoconductive drum 1Y is drivento rotate in a direction indicated by an arrow mark by the motor notillustrated. Then the charger 2Y uniformly charges the photoconductivedrum 1Y, and the exposing device 3Y exposes the photoconductive drum 1Ywith exposure light. By this, an electrostatic latent imagecorresponding to the yellow color component is formed on thephotoconductive drum 1Y. The electrostatic latent image on thephotoconductive drum 1Y is developed by the developer device 4Y usingyellow toner. A yellow image is formed on the photoconductive drum 1Y.

The yellow image on the photoconductive drum 1Y is conveyed to a primarytransfer nip portion where the primary transfer roller 7Y presses thephotoconductive drum 1Y through the intermediate transfer belt 6 whenthe photoconductive drum 1Y rotates in the direction indicated by thearrow mark. A primary transfer voltage is applied from a power sourceunit (not illustrated) to the primary transfer roller 7Y. By this, theyellow image on the photoconductive drum 1Y is transferred to theintermediate transfer belt 6 in the primary transfer nip portion.Furthermore, toner which remains in the photoconductive drum 1Y by isremoved by the drum cleaner 8Y.

Images formed by the image forming stations 10Y, 10M, 10C, and 10K aretransferred to the intermediate transfer belt 6. When the images aretransferred in an overlapping manner on the intermediate transfer belt6, a full-color image is formed on the intermediate transfer belt 6. Theimage held by the intermediate transfer belt 6 is transferred to thesecondary transfer nip portion T2. The sheet P is conveyed such that theimage on the intermediate transfer belt 6 is in contact with the sheet Pin the secondary transfer nip portion T2. The image on the intermediatetransfer belt 6 is transferred to the sheet P by the pair of secondarytransfer rollers 9 to which a secondary transfer voltage has beenapplied. Note that toner which is not transferred to the sheet P in thesecondary transfer nip portion T2 but remains in the intermediatetransfer belt 6 is removed by the belt cleaner 11.

The sheet P which holds the image is conveyed to the fixing device 100.The fixing device 100 applies heat and pressure on the sheet P whichholds the unfixed image so as to fix the unfixed image on the sheet P ina melting manner.

Next, a control block diagram of the image forming apparatus isillustrated in FIG. 2. A controller 303 is a control circuit whichcontrols various units. A ROM 90 stores various programs therein. Amemory 40 stores a look-up table (hereinafter referred to as a γ LUT)used to correct a tone characteristic and stores an image formingcondition, such as intensity of laser light of exposing devices 3Y, 3M,3C, and 3K. An image forming station 10 corresponds to the image formingstations 10Y, 10M, 10C, and 10K. The image forming station 10 has beendescribed hereinabove, and therefore, a description thereof is omittedhere.

A network interface card (NIC) unit 21 transmits image data suppliedthrough a network to a RIP unit 22 and transmits apparatus informationto an outside through the network. The RIP unit 22 analyzes image datadescribed by a page description language (PDL) and develops the imagedata. The RIP unit 22 outputs image area data of the image data anddensity signals of pixels in the image data (RGB data or CMYK data) inaccordance with a result of the analysis performed on the image data.

An image processor 60 performs various image processes on image data soas to correct the image data. The image processor 60 may be realized byan integrated circuit, such as an application specific integratedcircuit (ASIC), or may be realized when a CPU of the controller 303corrects image data in accordance with a program stored in advance.

Examples of the image data input to the image processor 60 includes RGBdata obtained by digitalizing three color components, that is, R (red),G (green), and B (blue), and CMYK data obtained by digitalizing fourcolor components, that is, C (cyan), M (magenta), Y (yellow), and K(black). An output direct mapping unit 61 converts RGB data into CMYKdata in a case where the RIP unit 22 transfers RGB data and image areadata to the image processor 60.

A γ correction unit 62 corrects the tone characteristic of image data.An image formed by the image forming apparatus does not have desireddensity. Therefore, the γ correction unit 62 corrects an input value (animage signal value) of image data so that an image formed by the imageforming apparatus has desired density. The γ correction unit 62 correctsthe tone characteristic of the image data (CMYK data) in accordance witha γ LUT_A and a γ LUT_B stored in the memory 40. Note that the γ LUT_Aand the γ LUT_B are stored in the memory 40 for each color component.The γ LUT_A and the γ LUT_B are tone correction tables which correct aninput value of image data.

Here, the γ LUT_A and the γ LUT_B will be now described. The γ LUT_A isa correction condition for correcting a printer characteristic of theimage forming apparatus to an ideal tone characteristic in a case wherethe image forming apparatus operates in a predetermined environmentalcondition and in a standard state of a predetermined charge amount ofthe developing agent. The γ LUT_A is determined through an experiment inadvance. However, the printer characteristic of the image formingapparatus changes depending on temperature and humidity around the imageforming apparatus, the number of formed images, a charge amount of thedeveloping agent, and the like. Therefore, density of an image formed bythe image forming apparatus changes depending on temperature andhumidity around the image forming apparatus, the number of formedimages, a charge amount of the developing agent, and the like.

Therefore, the image forming apparatus includes the γ LUT_B used tocorrect the γ LUT_A. The γ LUT_B is correction data which corrects theimage data corrected in accordance with the γ LUT_A so as to obtainimage data suitable for a state of the image forming apparatus currentlyused. The γ LUT_B obtains a printer characteristic of the current imageforming apparatus and is changed in accordance with the printercharacteristic. In a case where an image based on image data is to beformed, the γ correction unit 62 corrects the image data in accordancewith a γ LUT obtained by combining the γ LUT_A and the γ LUT_B.

A halftone processor 63 performs screening suitable for image area dataon image data (CMYK data) corrected by the γ correction unit 62. Bythis, the image data (CMYK data) which is multivalued data forindividual pixels is converted into binary data for individual pixels.For example, the screening is performed using a dither matrix so that acharacter region is clearly printed. For example, the screening isperformed on a photographic image region using an error diffusion methodso that moire is suppressed. Since the screening is a general technique,a detailed description thereof is omitted.

Image data converted by the image processor 60 is transferred to anexposing device 3 of the image forming station 10. The exposing device 3of the image forming station 10 is controlled based on the image dataconverted by the image processor 60. The exposing device 3 exposes thephotoconductive drum 1 so as to form an electrostatic latent image basedon the image data on the photoconductive drum 1. Since the image formingoperation is described hereinabove, a description thereof is omittedhere.

An operation unit 80 includes a power switch of the image formingapparatus, a mode selection button for selecting a mode of the imageforming apparatus, a numeric keypad, a determination button, and aliquid crystal screen. The liquid crystal screen displays information onan amount of remaining toner accommodated in the developer devices 4Y,4M, 4C, and 4K and an image associated with image data.

The density detection sensor 5 includes an LED 51 and photodiodes 52 and53. In the density detection sensor 5, the LED 51 irradiates ameasurement image with light and the photodiodes 52 and 53 receive thelight reflected by a measurement image. The photodiodes 52 and 53 outputsensor output values (voltage values) in accordance with intensity ofthe reflection light from the measurement image.

Note that the LED 51 functions as an irradiation unit which irradiatesthe measurement image with light. The photodiodes 52 and 53 function aslight receiving units which receive reflection light from themeasurement image.

A pattern generator 70 generates measurement image data for forming ameasurement image. The pattern generator 70 outputs pattern image datain a case where the tone characteristic of the image forming station 10is to be corrected. Furthermore, the pattern generator 70 outputs testimage data in a case where a conversion table 55 b is to be corrected.Note that automatic tone correction for correcting the tonecharacteristic of the image forming station 10 will be described indetail with reference to FIGS. 6 to 8, and visual correction forcorrecting the conversion table 55 b will be described in detail withreference to FIGS. 9 to 12.

An A/D conversion circuit 56 converts a sensor output value (a voltagevalue) of the density detection sensor 5 into a sensor output value (adigital signal) of a level in a range from 0 to 255. A densityconversion circuit 55 converts the sensor output value (the digitalsignal) into density (print density) of a measurement image on the sheetP in accordance with a conversion table. The density conversion circuit55 includes a conversion table 55 a and the conversion table 55 b.

The conversion table 55 a is determined in advance by an experiment.Therefore, if the correspondence relationship between a result ofmeasurement of a measurement image performed by the density detectionsensor 5 and print density is changed, the print density of themeasurement image may not be estimated with high accuracy only using theconversion table 55 a. Accordingly, the density conversion circuit 55additionally includes the conversion table 55 b for correcting a resultof conversion performed using the conversion table 55 a to compensatefor the accuracy of the density conversion circuit 55. The conversiontables 55 a and 55 b correspond to conversion conditions for convertinga sensor output value into density.

A conversion table updating unit 302 updates the conversion table 55 bin accordance with density of a test image on the sheet P visuallydetermined by the user and density of the test image measured by thedensity detection sensor 5. A density conversion table 81 stores dataindicating the correspondence relationships between density levels ofsample images in a sample chart illustrated in FIG. 9 and identificationnumbers of the sample images. The density conversion table 81 outputsdensity corresponding to an identification number input by the operationunit 80 to the conversion table updating unit 302. In this way, theconversion table updating unit 302 may obtain density of a test image onthe sheet P which is visually determined by the user.

A γ LUT generation unit 301 corrects the γ LUT_B in accordance with aresult of measurement of a pattern image on the intermediate transferbelt 6 and combines the γ LUT_A and the γ LUT_B so as to generate a γLUT_C.

A γ LUT setting unit 65 sets a γ LUT used by the γ correction unit 62 tocorrect image data and a γ LUT used by the γ correction unit 62 tocorrect measurement image data. In a case where the image formingapparatus forms an image based on image data input by the PC, thescanner, or the like, not illustrated, the γ LUT setting unit 65 setsthe γ LUT_C obtained by combining the γ LUT_A and the γ LUT_B to the γcorrection unit 62. Furthermore, in a case where the automatic tonecorrection is executed to correct the γ LUT_B, the γ LUT setting unit 65sets the γ LUT_A to the γ correction unit 62.

Hereinafter, three characteristic control operations of the imageforming apparatus will be described. As a first control operation,operations of the units performed in a case where the image formingapparatus forms an image based on image data supplied from the PC, thescanner, or the like, not illustrated will be described. When the imagedata is input, the γ LUT setting unit 65 sets the γ LUT_C obtained bycombining the γ LUT_A and the γ LUT_B to the γ correction unit 62. The γcorrection unit 62 corrects the image data in accordance with the γLUT_C. By the process described above, the image forming apparatus formsan image based on the image data on the sheet P.

As a second control operation, operations of the units performed in acase where the automatic tone correction for correcting the γ LUT_B isexecuted will be described. The pattern generator 70 transfers patternimage data to the γ correction unit 62. The γ LUT setting unit 65 setsthe γ LUT_A stored in the memory 40 in advance to the γ correction unit62. The γ correction unit 62 corrects the pattern image data inaccordance with the γ LUT_A. Then the image forming station 10 forms apattern image on the intermediate transfer belt 6. The density detectionsensor 5 measures the pattern image on the intermediate transfer belt 6.A sensor output value of the density detection sensor 5 is convertedinto a digital signal by the A/D conversion circuit 56. The densityconversion circuit 55 converts the digital signal into print density inaccordance with both of the conversion tables 55 a and 55 b. Then a γLUT generation unit 301 corrects the γ LUT_B so that the print densityconverted by the density conversion circuit 55 corresponds to targetdensity and generates the γ LUT_C by combining the γ LUT_A and the γLUT_B.

As a third control operation, operations of the units performed in acase where visual correction for correcting the conversion table 55 b isexecuted will be described. The pattern generator 70 transfers testimage data to the γ correction unit 62. The γ LUT setting unit 65 setsthe γ LUT_C to the γ correction unit 62. The γ correction unit 62corrects the test image data in accordance with the γ LUT_C. Then theimage forming station 10 forms a test image on the sheet P and the imageforming apparatus outputs a test sheet.

The pattern generator 70 transfers the test image data to the γcorrection unit 62. The γ correction unit 62 corrects the test imagedata in accordance with the γ LUT_C. Then the image forming station 10forms a test image on the intermediate transfer belt 6. The densitydetection sensor 5 measures the test image on the intermediate transferbelt 6. A sensor output value of the density detection sensor 5 isconverted into a digital signal by the A/D conversion circuit 56. Thedensity conversion circuit 55 converts the digital signal into printdensity in accordance with the conversion table 55 a. Thereafter, theconversion table updating unit 302 changes the conversion table 55 b inaccordance with the print density converted by the density conversioncircuit 55 and density information input by the user using the operationunit 80.

Next, a configuration of the density detection sensor 5 included in theimage forming apparatus will be described with reference to FIG. 3. Thedensity detection sensor 5 includes a case unit 50 which accommodatesthe LED 51, the photodiodes 52 and 53, an electric substrate (notillustrated), and various elements and a window portion 54 formed on thecase unit 50. The density detection sensor 5 may further include anoptical element, such as a lens.

The LED 51 is a light emitting element which irradiates a measurementimage formed on the intermediate transfer belt 6 with light. Awavelength of the light emitted from the LED 51 is in a range from 800to 850 nm, for example, taking spectral reflectance of the toner intoconsideration. The light is emitted from the LED 51 so as to be inclinedby an angle of 45 degrees relative to a direction orthogonal to asurface of the intermediate transfer belt 6.

The photodiode 52 is disposed on a virtual line which is inclined by anangle of 45 degrees relative to the direction orthogonal to the surfaceof the intermediate transfer belt 6. For example, the LED 51 and thephotodiode 52 are disposed in symmetric positions with respect to asurface which is orthogonal to the surface of the intermediate transferbelt 6. The photodiode 52 receives specular reflection light from ameasurement image on the intermediate transfer belt 6. The photodiode 52outputs a sensor output value (a voltage value) in accordance withintensity of the reflection light from the measurement image.

The photodiode 53 is disposed in a certain position so as not to receivethe specular reflection light from the intermediate transfer belt 6. Thephotodiode 53 is disposed on a virtual line which is inclined by anangle of 20 degrees, for example, relative to the direction orthogonalto the surface of the intermediate transfer belt 6. The photodiode 53receives irregular reflection light from the measurement image on theintermediate transfer belt 6. The photodiode 53 outputs a sensor outputvalue (a voltage value) in accordance with intensity of the reflectionlight from the measurement image.

The density detection sensor 5 measures, in a case where density of ameasurement image of black is to be measured, specular reflection lightfrom the measurement image. Therefore, in a case where the densitydetection sensor 5 detects the density of the measurement image ofblack, the density conversion circuit 55 converts a sensor output valueof the photodiode 52 into density. Meanwhile, the density detectionsensor 5 measures, in a case where density of a measurement image ofyellow, density of a measurement image of magenta, and density of ameasurement image of cyan are to be measured, irregular reflection lightfrom measurement images. Therefore, in a case where the densitydetection sensor 5 detects the density of the measurement image ofyellow, the density of the measurement image of magenta, the density ofthe measurement image of cyan, the density conversion circuit 55converts a sensor output value of the photodiode 53 into density.

Note that one of the sensor output values of the photodiodes 52 and 53is used to determine print density of a measurement image. However, theconversion into the print density may be performed in accordance withboth of the sensor output values of the photodiodes 52 and 53.

The density detection sensor 5 measures a measurement image on theintermediate transfer belt 6. Therefore, the density conversion circuit55 converts a result of the measurement performed by the densitydetection sensor 5 into density (print density) of a measurement imageon the sheet P in accordance with a conversion table. A case where thedensity detection sensor 5 measures a measurement image of black on theintermediate transfer belt 6 and the density conversion circuit 55converts a sensor output value into print density will be describedhereinafter.

The LED 51 irradiates the intermediate transfer belt 6 with light. Aregion on which the light from the LED 51 is incident corresponds to ameasurement position. While a measurement image (black) on theintermediate transfer belt 6 passes the measurement position describedabove, the photodiode 52 receives reflection light from the measurementimage (black). A sensor output value (a voltage value) output from thephotodiode 52 while the photodiode 52 receives the reflection light fromthe measurement image (black) corresponds to density of the measurementimage (black).

After the A/D conversion circuit 56 converts the sensor output value(the voltage value) into a sensor output value (a digital signal) of 8bits, the density conversion circuit 55 converts the sensor output valueof the photodiode 52 into a density Dblack of the measurement image ofblack. Note that the density Dblack corresponds to density data of themeasurement image of black.

In a case where the relationship between density of measurement imageformed on the sheet P and sensor output value is changed, for example,the density of the measurement image may not be detected with highaccuracy even if the density conversion circuit 55 converts the sensoroutput value in accordance with the conversion table 55 a. To compensatefor a result of the high-accuracy measurement of the measurement image,the density conversion circuit 55 converts the sensor output value intoprint density in accordance with both of the conversion tables 55 a and55 b. Note that the conversion table 55 b which is stored in the memory40 in advance before the visual correction is performed is data in whichdensity before conversion and density after conversion are the same aseach other. The conversion table 55 b functions as correction data forcorrecting the conversion table 55 a so as to compensate for a result ofthe high-accuracy measurement of the measurement image when the visualcorrection is performed.

Furthermore, in a case where the density detection sensor 5 measures ameasurement image of yellow, the controller 303 converts a sensor outputvalue of the photodiode 53 into print density of the measurement imageof yellow in accordance with a conversion table corresponding to themeasurement image of yellow. Similarly, in a case where the densitydetection sensor 5 measures a measurement image of magenta, thecontroller 303 converts a sensor output value of the photodiode 53 intoprint density of the measurement image of magenta in accordance with aconversion table corresponding to the measurement image of magenta.Similarly, in a case where the density detection sensor 5 measures ameasurement image of cyan, the controller 303 converts a sensor outputvalue of the photodiode 53 into print density of the measurement imageof cyan in accordance with a conversion table corresponding to themeasurement image of cyan. The memory 40 stores the conversion table 55a and the conversion table 55 b in advance.

FIG. 4 is a graph illustrating the conversion table 55 a. In FIG. 4, therelationship between a sensor output value of the density detectionsensor 5 corresponding to a measurement image and print density of themeasurement image in a case where density of the measurement image onthe intermediate transfer belt 6 is changed in a step-by-step manner inaccordance with area coverage modulation is illustrated. If an areacovering ratio of the toner of the measurement image is increased,density of the measurement image on the sheet P is also increased. Ifthe area covering ratio of the toner is increased, an amount ofreflection light from the measurement image is reduced. As an amount oflight received by a photodiode is reduced, a sensor output value isreduced. Therefore, as the sensor output value is reduced, the densityof the measurement image on the sheet P is increased. On the other hand,if the area covering ratio of the toner of the measurement image isreduced, the density of the measurement image on the sheet P is alsoreduced. If the area covering ratio of the toner is reduced, the amountof reflection light from the measurement image is increased. As theamount of light received by a photodiode is increased, a sensor outputvalue is increased. Therefore, as the sensor output value is increased,the density of the measurement image on the sheet P is reduced.

Tone Correction Control

FIG. 5A is a graph illustrating the printer characteristic of the imageforming apparatus. FIG. 5B is a graph illustrating the γ LUT used tocorrect the printer characteristic of the image forming apparatus ofFIG. 5A. Dotted lines in FIGS. 5A and 5B indicate an ideal tonecharacteristic. It is assumed that, in a description below, printdensity is proportional to an input value of image data in the idealtone characteristic (the dotted lines).

As illustrated in FIG. 5A, in a case where the image forming apparatusforms an image in accordance with an image signal X, the image has adensity Dy. However, an image to be formed by the image formingapparatus in accordance with the image signal X has a density Dx.Therefore, the γ correction unit 62 is required to correct the imagesignal X so that the image formed in accordance with the image signal Xhas the density Dx. Here, the γ LUT for correcting the image signal X isgenerated as follows. First, the relationship between the relationshipbetween the image signal X and the target density Dx and an image signalX′ corresponding to the image having the target density Dx isdetermined. Thereafter, data for correcting the image signal X into theimage signal X′ is generated in accordance with the relationshipdescribed above. The generated data corresponds to the γ LUT_A.

However, the printer characteristic of the image forming apparatuschanges in a case where the image forming apparatus forms a plurality ofimages or in a case where temperature or humidity around the imageforming apparatus changes. Therefore, the image forming apparatus formsa pattern image on the intermediate transfer belt 6 in a case where acertain condition is satisfied and updates the γ LUT in accordance witha result of measurement performed on the pattern image. This operationcorresponds to the automatic tone correction.

Examples of the certain condition for executing the automatic tonecorrection include a time immediately after a main power of the imageforming apparatus is turned on and the number of pages of images printedafter preceding automatic tone correction is executed which is largerthan a predetermined number of pages. Note that the image formingapparatus may be configured such that the controller 303 executes theautomatic tone correction in a case where a user inputs a command forinstructing execution of the automatic tone correction using theoperation unit 80.

In a case where an image is to be formed in accordance with image datatransferred from the PC or the scanner, not illustrated, the γcorrection unit 62 corrects the image data in accordance with the γLUT_C obtained by combining the γ LUT_A and the γ LUT_B. The γ LUT_B iscorrection data for correcting the γ LUT_A to generate the γ LUT_Csuitable for a current printer characteristic. The controller 303controls the image forming station 10 so as to form a pattern image onthe intermediate transfer belt 6. Then the γ LUT generation unit 301updates the γ LUT_B in accordance with a result of measurement performedon the pattern image by the density detection sensor 5 and combines theγ LUT_A and the γ LUT_B so as to generate the γ LUT_C.

FIGS. 6A to 6C are diagrams schematically illustrating a state in whichthe γ LUT_B is updated. FIG. 6A is a graph illustrating an ideal densitycharacteristic. FIG. 6B is a graph illustrating a density characteristicindicating the relationship between a value of a signal input by animage forming unit at a time when the pattern image is formed on theintermediate transfer belt 6 and a result of measurement performed bythe density detection sensor 5 on the pattern image. Note that thesignal input by the image forming unit represents a corrected imagesignal. Specifically, in FIG. 6B, the signal input by the image formingunit obtained at a time when the pattern image is formed corresponds toa signal value obtained when the γ correction unit 62 corrects an inputvalue of pattern image data in accordance with the γ LUT_A. The patternimage includes nine pattern images and the different pattern imagescorrespond to different input values.

In FIG. 6B, white circles indicate target density values correspondingto the value of the signal input by the image forming unit and blackcircles indicate density values detected by the density detection sensor5. A measurement result DO on the surface of the intermediate transferbelt 6 on which any pattern image is not formed remains to be 0 (a fixedvalue) even if the printer characteristic of the image forming apparatusis changed. Furthermore, although a pattern image corresponding to amaximum value of the value of the signal input by the image forming unitis not formed, a density value of the pattern image corresponding to themaximum value of the value of the signal input by the image forming unitis estimated from results of measurement of the nine pattern images.

The reason that a pattern image of high density is not formed is thatthe density detection sensor 5 is not capable of measuring a change of atoner applying amount of the pattern image of high density with highaccuracy. Since the image forming apparatus forms pattern images by anarea coverage modulation method, although the toner applying amount ischanged, change of a reflection light amount is negligible in a highdensity region in which the toner applying amount of the pattern imagescovers the intermediate transfer belt 6. Therefore, the γ LUT generationunit 301 estimates an estimation density value Dmax of the pattern imagecorresponding to the maximum value of the signal input by the imageforming unit in accordance with a result of the measurement performed onthe pattern images by the density detection sensor 5. For example, the γLUT generation unit 301 extrapolates a measurement result (theestimation density value Dmax) of the pattern image corresponding to themaximum value of the signal input by the image forming unit inaccordance with a result of measurement performed on a pattern imagehaving the highest density in the pattern images of nine tones and aresult of measurement performed on a pattern image having the secondhighest density.

Next, a method for generating the γ LUT_B will be described. To converta density characteristic (a solid line) of FIG. 6B into an ideal densitycharacteristic (a dotted line) of FIG. 6B, a signal input by the imageforming unit is replaced by a signal input by the image forming unit forforming a pattern image having target density corresponding to thesignal input by the image forming unit. In this way, the γ LUT_B forcorrecting the signal input by the image forming unit is generated. Notethat the γ LUT_B stored in the memory 40 in advance is data which doesnot change the signal input by the image forming unit. γ LUT_B iscorrected when the automatic tone correction is executed.

In FIG. 6C, a graph (a dotted line) of the γ LUT_B before the correctionand a graph (a solid line) of the γ LUT_B after the correction(represented by the γ LUT_B′ in FIG. 6C) are illustrated. As illustratedin FIG. 6C, the γ LUT_B′ after the correction and the densitycharacteristic of FIG. 6B are line symmetrical with respect to the idealdensity characteristic (the dotted line).

Hereinafter, the automatic tone correction for correcting the γ LUT_Bwill be described with reference to a flowchart of FIG. 7. Note that, inthe automatic tone correction, the image forming station 10 forms apattern image on the intermediate transfer belt 6 and the γ LUT_B isautomatically corrected in accordance with a result of measurementperformed by the density detection sensor 5 on the pattern image. Thatis, the user does not perform any operation in the automatic tonecorrection.

The controller 303 executes the automatic tone correction illustrated inFIG. 8 in accordance with a program stored in the ROM 90. When theautomatic tone correction is executed, the controller 303 forms patternimages on the intermediate transfer belt 6 (S100). The controller 303causes the pattern generator 70 to output pattern image data. Thecontroller 303 causes the γ correction unit 62 to correct the patternimage data in accordance with the γ LUT_A. The image forming station 10forms pattern images of nine tones of different density levels on theintermediate transfer belt 6 in accordance with the pattern image datacorrected by the γ correction unit 62. In step S100, the image formingstation 10 functions as the image forming unit which forms patternimages. The intermediate transfer belt 6 corresponds to an image-bearingmember which carries the pattern images.

FIG. 8 is a diagram schematically illustrating the pattern images formedon the intermediate transfer belt 6. The pattern images of nine toneshave different density levels for each color. Specifically, 36measurement images are formed in total for yellow, magenta, cyan, andblack. A length of one pattern image is 25 mm in a conveying directionof the intermediate transfer belt 6 and 15 mm in a direction orthogonalto the conveying direction, for example. The nine pattern images areformed in accordance with 8-bit signal values of 9, 36, 64, 90, 117,144, 171, 200, and 225, for example.

Subsequently, the controller 303 measures the pattern images by thedensity detection sensor 5 (S101). In step S101, a sensor output valueis output every 2 msec while the pattern images pass the measurementposition of the density detection sensor 5. The density detection sensor5 performs measurement 25 times on each of the pattern images. The A/Dconversion circuit 56 converts the sensor output value into a sensoroutput value of a digital signal. The controller 303 averages 23 sensoroutput values, except for a maximum value and a minimum value, in 25sensor output values. Then the controller 303 causes the densityconversion circuit 55 to convert an average value of the sensor outputvalues into print density in accordance with both of the conversiontables 55 a and 55 b.

The controller 303 updates the γ LUT_B in accordance with the printdensity of the pattern images measured in step S101 (step S102). Thecontroller 303 causes the γ LUT generation unit 301 to determine adensity characteristic in accordance with a result of the print densitymeasured in step S101. Then the γ LUT generation unit 301 updates the γLUT_B as described with reference to FIG. 6. The controller 303 causesthe γ LUT generation unit 301 to combine the γ LUT_A and the γ LUT_B soas to generate the γ LUT_C (S103), and thereafter, terminates theautomatic tone correction.

Visual Correction

In a case where toner or paper dust adheres to the window portion 54 ofthe density detection sensor 5, a sensor output value includes an error.This is because, in a case where toner or paper dust adheres to thewindow portion 54, intensity of light emitted to a measurement imagefrom the LED 51 is reduced or intensity of reflection light received bythe photodiodes 52 and 53 is reduced. In the case where toner or paperdust adheres to the window portion 54, print density of a measurementimage may not be detected with high accuracy since sensor output valuesof the photodiodes 52 and 53 include errors.

Furthermore, in a case where the surface of the intermediate transferbelt 6 has roughness since a large number of images are formed, thecorrespondence relationship between a sensor output value of ameasurement image on the intermediate transfer belt 6 and print densityof the measurement image changes. In the case where the surface of theintermediate transfer belt 6 has roughness, reflection light from thesurface of the intermediate transfer belt 6 changes. Therefore, inparticular, a sensor output value obtained when a measurement image of alow density corresponding to a low covering ratio of toner is measuredincludes an error. In the case where the surface of the intermediatetransfer belt 6 has roughness, print density of a measurement image maynot be detected with high accuracy since sensor output values of thephotodiodes 52 and 53 include errors.

Furthermore, a rate (a transfer efficiency) of toner transferred fromthe intermediate transfer belt 6 to the sheet P in the secondarytransfer nip portion T2 changes depending on temperature or humidityaround the image forming apparatus. Therefore, in a case where thetemperature or the humidity around the image forming apparatus changes,it is likely that the relationship between an amount of toner of ameasurement image formed on the intermediate transfer belt 6 and density(print density) of the measurement image formed on the sheet P changes.

Furthermore, the rate (the transfer efficiency) of toner transferredfrom the intermediate transfer belt 6 to the sheet P in the secondarytransfer nip portion T2 changes also in a case where the pair ofsecondary transfer rollers 9 is deteriorated with time. This is becausea resistance value of the pair of secondary transfer rollers 9 changessince the pair of secondary transfer rollers 9 is deteriorated withtime. Specifically, even in the case where the pair of secondarytransfer rollers 9 changes with time, it is likely that the relationshipbetween the amount of toner of the measurement image formed on theintermediate transfer belt 6 and the density (the print density) of themeasurement image formed on the sheet P changes.

Therefore, since the correspondence relationship between the sensoroutput value and the print density is different from the presetcorrespondence relationship if the transfer efficiency changes, thedensity detection sensor 5 may not detect the print density of themeasurement image with high accuracy.

Accordingly, to detect the print density of the measurement image withhigh accuracy in accordance with a result of the measurement performedon the measurement image on the intermediate transfer belt 6, theconversion table updating unit 302 changes the conversion table 55 b inaccordance with density information of the measurement image formed onthe sheet P and the sensor output value of the measurement image. Bythis, even in a case where the window portion 54 of the densitydetection sensor 5 gets dirty, the conversion table 55 b reduces ameasurement error of the density detection sensor 5, and accordingly, aresult of high-accuracy measurement performed on the measurement imagemay be compensated for. Furthermore, even in the case where the surfaceof the intermediate transfer belt 6 has roughness, the conversion table55 b reduces a measurement error of the density detection sensor 5, andaccordingly, a result of high-accuracy measurement performed on themeasurement image may be compensated for. Furthermore, even in a casewhere the transfer efficiency changes, the conversion table 55 bconverts a sensor output value into print density of the measurementimage with high accuracy, and accordingly, the print density of themeasurement image may be detected with high accuracy.

The visual correction for updating the conversion table 55 b will bedescribed hereinafter.

FIG. 9 is a diagram schematically illustrating a sample chart used inthe visual correction. In the sample chart, 10 sample images for eachcolor are printed. Furthermore, numbers (1 to 10) printed beside thesample images are identification numbers to be used by the user toidentify the sample images.

The sample chart is a color sample for identifying density of a testimage on a test sheet printed by the image forming apparatus in thevisual correction. Therefore, density levels of the sample images areadjusted in advance. The correspondence relationship between theidentification numbers and the density levels in the density conversiontable 81 (FIG. 2) is the same as the correspondence relationship betweenthe identification numbers of the sample images printed on the samplechart and the density levels of the sample images.

FIG. 10 is a diagram schematically illustrating a test sheet printed bythe image forming apparatus when the visual correction is executed. Thetest sheet is the sheet P on which test images are formed as measurementimages. One test sheet includes test images of cyan, test images ofmagenta, test images of yellow, and test images of black formed thereon.

The test sheet includes test images of different density levels, thatis, test images for low density (A column), test images for middledensity (B column), and test images for high density (C column). Thetest images for low density are formed in accordance with an imagesignal value corresponding to a target density level of 0.4, forexample. The test images for middle density are formed in accordancewith an image signal value corresponding to a target density level of0.8, for example. The test images for high density are formed inaccordance with an image signal value corresponding to a target densitylevel of 1.2, for example.

In the visual correction, the user compares the test image on the testsheet with the sample images in the sample chart. Subsequently, the userdetermines one of the sample images having a density level most similarto the density level of the test image. Then the user inputs anidentification number of the sample image having the density level whichis most similar to the density level of the test image in the operationunit 80 in accordance with guidance displayed in the liquid crystalscreen of the operation unit 80.

For example, in a case where a density level of one of the sample imageswhich is indicated by an identification number “7” is most similar to adensity level of one of the test images of the middle density, the userinputs “7” using the numerical keypad of the operation unit 80. Thedensity conversion table 81 outputs the density level of the sampleimage corresponding to the identification number. The operation unit 80inputs information on the density level of the test image visuallydetermined by the user. The identification number input from theoperation unit 80 corresponds to information on the density level of thetest image (a user instruction). Specifically, the conversion tableupdating unit 302 receives the user instruction based on a result of thecomparison between the sample images and the test image performed by theuser from the operation unit 80.

Operations performed by the various units included in the image formingapparatus in a case where the visual correction is executed will now bedescribed with reference to a flowchart of FIG. 11. The visualcorrection is executed when a command for instructing execution of thevisual correction is input by the user operating the mode selectionbutton of the operation unit 80. Note that the controller 303 executesthe visual correction illustrated in FIG. 11 by controlling the variousunits included in the image forming apparatus in accordance with aprogram stored in the ROM 90.

First, the image forming apparatus starts print of test sheets (S131).In step S131, a γ LUT setting unit 65 sets the γ LUT_C stored in thememory 40 to the γ correction unit 62. The pattern generator 70 outputstest image data. The γ correction unit 62 corrects the test image datain accordance with the γ LUT_C. The image forming station 10 forms testimages on the intermediate transfer belt 6 in accordance with thecorrected test image data. The pair of secondary transfer rollers 9 andthe power source unit transfer the test images on the intermediatetransfer belt 6 to the sheet P, the fixing device 100 fixes the testimages on the sheet P, and a conveying roller, not illustrated,discharges the test sheets from the image forming apparatus.

In step S131, the image forming station 10 functions as the imageforming unit which forms the test images. The pair of secondary transferrollers 9 functions as a transfer unit which transfers the test imageson the intermediate transfer belt 6 to the sheet P. The fixing device100 functions as a fixing unit which fixes the test images on the sheetP. The γ correction unit 62 functions as a correction unit whichcorrects the test image data in accordance with the γ LUT. Thecontroller 303 functions as a control unit which causes the γ correctionunit 62 to correct the test image data and which causes the imageforming station 10 to form the test images on the sheet P.

Next, the image forming apparatus forms test images on the intermediatetransfer belt 6 (S132). The pattern generator 70 outputs test image datato the γ correction unit 62. The γ correction unit 62 corrects the testimage data in accordance with the γ LUT_C. Here, an image formingcondition for forming test images is the same as the image formingcondition for forming the test sheets in step S131. Note that the imageforming condition includes charge voltages of the chargers 2Y, 2M, 2C,and 2K, light intensity of laser light of the exposing devices 3Y, 3M,3C, and 3K, and transfer voltages applied to the primary transferrollers 7Y, 7M, 7C, and 7K.

The image forming station 10 functions as the image forming unit whichforms the test images on the intermediate transfer belt 6 in step S132.The γ correction unit 62 functions as a correction unit which correctsthe test image data in accordance with the γ LUT_C. The controller 303functions as a control unit which causes the γ correction unit 62 tocorrect the test image data and which causes the image forming station10 to form the test images on the intermediate transfer belt 6.

Subsequently, the density detection sensor 5 measures the test images onthe intermediate transfer belt 6 (S133). In step S133, the LED 51 startslight emission before the test images on the intermediate transfer belt6 reach the measurement position. The A/D conversion circuit 56 convertsa voltage value output from the photodiode 52 (or 53) into a digitalsignal at timings when the test images on the intermediate transfer belt6 pass the measurement position. The density conversion circuit 55converts the digital signal output from the A/D conversion circuit 56into a density value only in accordance with the conversion table 55 a.

Although the image forming station 10 forms the test images twice instep S131 and step S132, the test images may be formed only once. Inthis case, the density detection sensor 5 measures the test images onthe intermediate transfer belt 6 before the test images are transferredto the sheet P.

Subsequently, the image forming apparatus waits until information ondensity of one of the test images visually determined by the user isinput (S134). The image forming apparatus waits until an identificationnumber of a sample image having a density level which is most similar tothat of the test image is input using the operation unit 80. When theidentification number is input using the operation unit 80, the densityconversion table 81 outputs a density level of the sample imagecorresponding to the identification number as a density level of thetest image.

When the information on density levels of the test images visuallydetermined by the user are input to all the test images in step S131,the image forming apparatus updates the conversion table 55 b (S136).The conversion table updating unit 302 updates the conversion table 55 bsuch that the density levels of the test images converted using theconversion table 55 a in step S133 match density levels of the testimages output from the density conversion table 81 in step S134. Bythis, a conversion condition for converting a result of the measurementperformed by the density detection sensor 5 is generated. The imageforming apparatus then terminates the visual correction.

A method for updating the conversion table 55 b employed in theconversion table updating unit 302 will be described hereinafter withreference to FIG. 12. FIG. 12 is a diagram illustrating a graph (adotted line) of the conversion table 55 b before the correction and agraph (a solid line) of the conversion table 55 b after the correction.

The conversion table updating unit 302 generates a first region of theconversion table 55 b in accordance with Expressions 1 and 2. The firstregion corresponds to a range from a density value of 0 to a densityvalue DA of a test image A of low density determined using the densitydetection sensor 5 and the conversion table 55 a.D′=α1×D  Expression 1α1=(DA2−0)/(DA1−0)  Expression 2

Here, D denotes a density value before the conversion is performed inaccordance with the conversion table 55 b and D′ denotes a density valueafter the conversion is performed in accordance with the conversiontable 55 b. Furthermore, α1 denotes a correction coefficient used by theconversion table 55 b to correct density values in the first region. DA1denotes a density value of the test image A on the intermediate transferbelt 6 measured by the density detection sensor 5. DA2 denotes a densityvalue of the test image A on the sheet P visually determined by theuser. DA2 is determined with reference to a density conversion table inaccordance with the identification number input using the operation unit80.

The conversion table updating unit 302 divides the density value of thetest image A visually determined by the user by the density value of thetest image A measured by the density detection sensor 5 so as tocalculate the correction coefficient α1 of the first region. By this,the conversion table 55 b which is used to correct a shift between theprint density obtained by visual check by the user and the density valuedetermined using the density detection sensor 5 and the conversion table55 a is generated in the first region.

The conversion table updating unit 302 generates a second region of theconversion table 55 b in accordance with Expressions 3, 4, and 5. Thesecond region corresponds to a range from the density value DA of thetest image A of low density determined using the density detectionsensor 5 and the conversion table 55 a to a density value DB of a testimage B of middle density determined using the density detection sensor5 and the conversion table 55 a.D′=α2×D+β1  Expression 3α2=(DB2−DA2)/(DB1−DA1)  Expression 4β1=DB2−α2×DB1  Expression 5

Here, D denotes a density value before the conversion is performed inaccordance with the conversion table 55 b and D′ denotes a density valueafter the conversion is performed in accordance with the conversiontable 55 b. Furthermore, α2 and β1 denote correction coefficients usedby the conversion table 55 b to correct a density value in the secondregion. DB1 denotes a density value of the test image B on theintermediate transfer belt 6 measured by the density detection sensor 5.DB2 denotes a density value of the test image B on the sheet P visuallydetermined by the user. DB2 is determined with reference to the densityconversion table in accordance with an identification number input usingthe operation unit 80.

The conversion table updating unit 302 divides the density value of thetest image B visually determined by the user by the density value of thetest image B measured by the density detection sensor 5 so as tocalculate the correction coefficient α2 of the second region. Theconversion table updating unit 302 calculates the correction coefficientβ1 in accordance with the correction coefficient α2, the density valueof the test image B measured by the density detection sensor 5, and thedensity value of the test image B visually determined by the user. Bythis, the conversion table 55 b which is used to correct a shift betweenthe print density obtained by visual check by the user and the densityvalue determined using the density detection sensor 5 and the conversiontable 55 a is generated in the second region. Note that the conversiontable updating unit 302 may calculate the correction coefficient β1 inaccordance with the correction coefficient α2, the density value of thetest image A measured by the density detection sensor 5, and the densityvalue of the test image A visually determined by the user.

The conversion table updating unit 302 generates a third region of theconversion table 55 b in accordance with Expressions 6, 7, and 8. Thethird region corresponds to a range from the density value DB of thetest image B of middle density determined using the density detectionsensor 5 and the conversion table 55 a to a density value DC of a testimage C of high density determined using the density detection sensor 5and the conversion table 55 a.D′=α3×D+β2  Expression 6α3=(DC2−DB2)/(DC1−DB1)  Expression 7β2=DC2−α3×DC1  Expression 8

Here, D denotes a density value before the conversion is performed inaccordance with the conversion table 55 b and D′ denotes a density valueafter the conversion is performed in accordance with the conversiontable 55 b. Furthermore, α3 and β2 denote correction coefficients usedby the conversion table 55 b to correct a density value in the thirdregion. DC1 denotes a density value of the test image C on theintermediate transfer belt 6 measured by the density detection sensor 5.DC2 denotes a density value of the test image C on the sheet P visuallydetermined by the user. DC2 is determined with reference to the densityconversion table in accordance with an identification number input usingthe operation unit 80.

The conversion table updating unit 302 divides the density value of thetest image C visually determined by the user by the density value of thetest image C measured by the density detection sensor 5 so as tocalculate the correction coefficient α3 of the third region. Theconversion table updating unit 302 calculates the correction coefficientβ2 in accordance with the correction coefficient α3, the density valueof the test image C measured by the density detection sensor 5, and thedensity value of the test image C visually determined by the user. Bythis, the conversion table 55 b which is used to correct a shift betweenthe print density obtained by visual check by the user and the densityvalue determined using the density detection sensor 5 and the conversiontable 55 a is generated in the third region. Note that the conversiontable updating unit 302 may calculate the correction coefficient β2 inaccordance with the correction coefficient α3, the density value of thetest image B measured by the density detection sensor 5, and the densityvalue of the test image B visually determined by the user.

The conversion table updating unit 302 generates a fourth region of theconversion table 55 b in accordance with Expressions 9 and 10. Thefourth region corresponds to a range of density values higher than thedensity value of the test image C of high density determined using thedensity detection sensor 5 and the conversion table 55 a.D′=D+β3  Expression 9β3=DC2−DC1  Expression 10

Here, D denotes a density value before the conversion is performed inaccordance with the conversion table 55 b and D′ denotes a density valueafter the conversion is performed in accordance with the conversiontable 55 b. Furthermore, β3 denotes a correction coefficient used by theconversion table 55 b to correct a density value in the fourth region.DC1 denotes a density value of the test image C on the intermediatetransfer belt 6 measured by the density detection sensor 5. DC2 denotesa density value of the test image C on the sheet P visually determinedby the user. DC2 is determined with reference to the density conversiontable in accordance with an identification number input using theoperation unit 80.

The conversion table updating unit 302 calculates the correctioncoefficient β3 by subtracting the density value of the test image Cvisually determined by the user from the density value of the test imageC measured by the density detection sensor 5.

In step S136 of FIG. 11, the conversion table updating unit 302generates the conversion table 55 b by combining the first region of theconversion table 55 b, the second region of the conversion table 55 b,the third region of the conversion table 55 b, and the fourth region ofthe conversion table 55 b. The conversion table 55 b is stored in thememory 40.

For example, as information on density visually measured by the user, ina case where a density level of the test image A is 0.3, a density levelof the test image B is 0.7, and a density level of the test image C is1.3, the correction coefficients α1, α2, α3, β1, β2, and β3 aredetermined as follows.

In the first region corresponding to a density level before theconversion in a range from 0 to 0.4 inclusive, the correctioncoefficient α1 is ¾. In the second region corresponding to a densityvalue before the conversion in a range larger than 0.4 and equal to orsmaller than 0.8, the correction coefficient α2 is 1 and the correctioncoefficient β1 is −0.1. In the third region corresponding to a densityvalue before the conversion in a range larger than 0.8 and equal to orsmaller than 1.2, the correction coefficient α3 is 1.5 and thecorrection coefficient β2 is −0.5. In the fourth region corresponding toa density value before the conversion in a range larger than 1.2, thecorrection coefficient β3 is 0.1.

A conversion table updating unit 302 generates the conversion table 55 bin accordance with information on density of a test image visually inputby the user and a sensor output value of the density detection sensor 5.Therefore, a measurement error of the density detection sensor 5 may bereduced and a high-accuracy measurement result may be ensured.

Furthermore, the γ LUT_C is generated in accordance with a result ofmeasurement performed by the density detection sensor 5 on the patternimage. Therefore, if print density of a measurement image may bemeasured with high accuracy in accordance with the conversion table 55b, a density characteristic of an image formed by the image formingapparatus on the sheet P may be corrected with high accuracy.

Furthermore, in the foregoing description, examples of measurementimages formed in the visual correction correspond to the test images A,B, and C having different density levels. However, the number of testimages formed on the sheet P so as to correct the conversion table 55 bmay be arbitrarily set. For example, in a case where only one test imageis used, the conversion table 55 b is generated such that a densitylevel of 0 before the conversion is converted into a density level of 0after the conversion, and a density value converted using the densitydetection sensor 5 and the conversion table 55 a matches print densityvisually determined by the user. Assuming that a density value convertedusing the density detection sensor 5 and the conversion table 55 a isdenoted by V and print density visually determined by the user isdenoted by W, the conversion table updating unit 302 generates theconversion table 55 b in accordance with Expression 11.D′=(W/V)×D  Expression 11

Note that D denotes a density value after the conversion is performed inaccordance with the conversion table 55 a and D′ denotes a density valueafter the conversion is performed in accordance with the conversiontable 55 b. The conversion table 55 b is a function of an inclinationW/V which passes an intersection between the density value convertedusing the density detection sensor 5 and the conversion table 55 a andthe print density visually determined by the user.

Furthermore, although the γ LUT_C is used when the γ correction unit 62corrects the test image data in the description above, the γ LUT_Astored in advance may be used. For example, in a case where the printercharacteristic is considerably changed since the photoconductive drum 1and the developer device 4 of the image forming apparatus are changed,it is more possible that the conversion table 55 b is updated with highaccuracy when the γ LUT_A for correcting the printer characteristic whenthe image forming apparatus operates in a standard state is used.

Furthermore, in a case where the visual correction is executed, theautomatic tone correction may be executed before the image formingapparatus prints a test sheet. With this configuration, since the γLUT_C suitable for the printer characteristic of the image formingapparatus is generated, density of test images formed on the test sheetmay be controlled to be target density, and the conversion table 55 bmay be corrected with high accuracy.

Moreover, although the sample images in the sample chart and the testimages on the test sheet are measurement images having a rectangleshape, the shape is not limited to rectangle. The shape of the sampleimages and the test images may be triangle or a circle in which aportion thereof is cut out.

Hereinafter, characteristics of the sample images formed on the samplechart will be described. FIG. 13 is a graph illustrating colordifferences ΔE between adjacent sample images in the sample chartillustrated in FIG. 9. Density of a sample image corresponding to anidentification number 1 is the lowest and density of a sample imagecorresponding to an identification number 10 is the highest.

Among the sample images printed on the sample chart, the colordifferences ΔE between adjacent sample images satisfy a conditionrepresented by Expression 12.ΔElow>ΔEhigh≧1.6  Expression 12

In Expression 12, a color difference ΔElow of adjacent sample images ina sample image group of low density smaller than a density level of 1.2is larger than a color difference ΔEhigh of adjacent sample images in asample image group of high density equal to or larger than a densitylevel of 1.2. Furthermore, the color difference ΔEhigh of adjacentsample images in the sample image group of the high density is largerthan 1.6.

In a plurality of sample images printed on the sample chart, all colordifferences of adjacent sample images are 1.6 or more. This is because,if a color difference is 1.6 or more, human eyes may recognize adifference of image density. By this, the user may recognize adifference between a density level of any one of sample images of theidentification numbers 1 to 10 and an adjacent sample image thereof, andmay determine a sample image having density which is the most similar todensity of a test image with high accuracy.

Furthermore, a color difference ΔE between the sample images of theidentification numbers 8 and 9 and a color difference ΔE between thesample images of the identification numbers 9 and 10 are smaller thancolor differences ΔE of the other pairs of sample images which areadjacent to each other. This is because, an amount of change of densityrelative to an amount of change of a color difference ΔE betweenadjacent sample images of high density is larger than an amount ofchange of density relative to an amount of change of a color differenceΔE between adjacent sample images of low density. Therefore, colordifferences ΔE between adjacent sample images corresponding to theidentification numbers 1 to 8 are larger than 3. Accordingly, the numberof sample images to be printed on the sample chart may be reduced andthe number of sample images of high density to be printed on the samplechart may be increased. Note that, in FIG. 13, the color difference ΔEof the sample image of the identification number 1 represents a colordifference between the sample image of the identification number 1 and asheet on which the sample image is printed.

FIG. 14 is a graph illustrating density of the sample images printed onthe sample chart. A density difference between the sample images of theidentification numbers 8 and 9 and a density difference between thesample images of the identification numbers 9 and 10 are smaller than0.3.

According to an experiment, when an examinee checks an image of lowdensity of a density level of 0.4, a difference between densitydetermined by the examinee and actual density is ±0.1 or less. On theother hand, when the examinee checks an image of high density of adensity level of 1.4, a difference between density determined by theexaminee and actual density is 0.4 at maximum. Specifically, as imagedensity is increased, humans may not reliably distinguish image density.Therefore, the sample images of high density to be printed on the samplechart are required to reduce density differences between adjacent sampleimages.

Therefore, in the sample chart, as illustrated in FIG. 14, colordifferences ΔE between adjacent sample images in the high density islarger than 1.6 and smaller than color differences ΔE of adjacent sampleimages of low density. Since density differences between the sampleimages of the high density printed on the sample chart are smaller than0.3, the user may specify density of the test image with high accuracy.

FIGS. 15A and 15B are diagrams illustrating comparative examples in acase where a color difference ΔE between adjacent sample images ofyellow is set to 3.2. An axis of abscissa denotes an accumulated colordifference ΣΔE. As illustrated in FIG. 15A, all sample images of yellowfrom low density to high density of the comparative examples have colordifferences ΔE of 3.2. As illustrated in FIG. 15B, a maximum densitylevel of the sample images of yellow of the comparative examples is 1.9.A density difference between the sample images of the identificationnumbers 10 and 9 is 0.4. In a case where a color difference betweenadjacent sample images is set to 3.2, a density difference betweenadjacent images of high density is large, and therefore, the user maynot determine density of a test image of high density with highaccuracy.

Therefore, in a sample image group including the sample images of theidentification numbers 9 and 10, a color difference ΔE between theadjacent sample images is smaller than a threshold value of 3 and equalto or larger than 1.6. Furthermore, in a sample image group includingthe sample images of the identification numbers 1 to 7, colordifferences ΔE between the adjacent sample images are larger than thethreshold value of 3. Density of the sample image of the identificationnumber 9 or density of the sample image of the identification number 10is higher than density of one of the sample images of the identificationnumbers 1 to 7.

According to the present invention, the color differences ΔE of theadjacent sample images of the high density are smaller than the colordifferences ΔE of the adjacent sample images of the low density.Therefore, the density difference between the sample images of the highdensity is suppressed, and the user may specify density of a test imageof high density with high accuracy. Furthermore, the color differencesΔE between the adjacent sample images of the high density correspond toa density difference recognized by humans. Accordingly, the user mayspecify a sample image having density most similar to that of a testimage from among a plurality of sample images.

While the present invention has been described with reference toexemplary embodiments, it is to be understood that the invention is notlimited to the disclosed exemplary embodiments. The scope of thefollowing claims is to be accorded the broadest interpretation so as toencompass all such modifications and equivalent structures andfunctions.

This application claims the benefit of Japanese Patent Application No.2015-060149, filed Mar. 23, 2015, which is hereby incorporated byreference herein in its entirety.

What is claimed is:
 1. An image forming apparatus comprising: an imageprocessor configured to correct image data based on a correctioncondition; an image former configured to form an image based on thecorrected image data, using developer; an intermediate transfer memberon which the image former forms a measurement image; a sensor configuredto measure the measurement image on the intermediate transfer member; aconverter configured to convert a measurement result of the measurementimage by the sensor, based on a conversion condition; a gamma generatorconfigured to generate the correction condition based on the measurementresult converted by the convertor; a controller configured to controlthe image former to form a first test image based on test image data,control the sensor to measure the first test image on the intermediatetransfer member, and control the image former to form a second testimage, based on test image data, on a sheet; a receptor configured toreceive a user instruction based on a result of a user having compared asample image and the second test image formed on the sheet; and aconversion condition generator configured to generate the conversioncondition based on the user instruction and the measurement result ofthe first test image by the sensor, wherein the sample image includes aplurality of sample images arranged along a predetermined direction, theplurality of sample images is formed by using a developer of apredetermined color, the plurality of sample images includes a firstsample image, a second sample image, a third sample image and a fourthsample image, the first sample image and the second sample image areadjacent to each other in the predetermined direction, and the thirdsample image and the fourth sample image are adjacent to each other inthe predetermined direction, density of the first sample image is higherthan density of the second sample image, density of the second sampleimage is higher than density of the third sample image, density of thethird sample image is higher than density of the fourth sample image, acolor difference between the first sample image and the second sampleimage is smaller than a threshold value, and a color difference betweenthe third sample image and the fourth sample image is larger than thethreshold value.
 2. The image forming apparatus according to claim 1,wherein a color difference between the first and second sample images isequal to or larger than 1.6.
 3. The image forming apparatus according toclaim 1, wherein the threshold value is larger than
 3. 4. The imageforming apparatus according to claim 1, wherein the sample imageincludes identification information corresponding to the sample image,and the user instruction corresponds to the identification informationinput by the user.
 5. The image forming apparatus according to claim 1,wherein the correction condition corresponds to a tone correction tablefor correcting a tone characteristic of the image data.
 6. The imageforming apparatus according to claim 1, wherein the second test image isthe same as the first test image.
 7. An image forming apparatuscomprising: an image processor configured to correct image data based ona correction condition; an image former configured to form an imagebased on the corrected image data, using developer; an intermediatetransfer member on which the image former forms the image; a transferconfigured to transfer the image on the intermediate transfer member toa sheet; a sensor configured to measure the measurement image on theintermediate transfer member; a converter configured to convert ameasurement result of the measurement image by the sensor, based on aconversion condition; a gamma generator configured to generate thecorrection condition based on the measurement result converted by theconvertor; a controller configured to control the image former to form atest image, control the sensor to measure the test image on theintermediate transfer member, and control the transfer to transfer thetest image formed on the intermediate transfer member to the sheet; areceptor configured to receive a user instruction based on a result of auser having compared a sample image and the test image transferred onthe sheet; and a conversion condition generator configured to generatethe conversion condition based on the user instruction and themeasurement result of the test image by the sensor, wherein the sampleimage includes a plurality of sample images arranged along apredetermined direction, the plurality of sample images is formed byusing a developer of a predetermined color, the plurality of sampleimages includes a first sample image, a second sample image, a thirdsample image and a fourth sample image, the first sample image and thesecond sample image are adjacent to each other in the predetermineddirection, the third sample image and the fourth sample image areadjacent to each other in the predetermined direction, density of thefirst sample image is higher than density of the second sample image,density of the second sample image is higher than density of the thirdsample image, density of the third sample image is higher than densityof the fourth sample image, a color difference between the first sampleimage and the second sample image is smaller than a threshold value, anda color difference between the third sample image and the fourth sampleimage is larger than the threshold value.
 8. The image forming apparatusaccording to claim 7, wherein a color difference between the first andsecond sample images is equal to or larger than 1.6.
 9. The imageforming apparatus according to claim 7, wherein the threshold value islarger than
 3. 10. The image forming apparatus according to claim 7,wherein the sample image includes identification informationcorresponding to the sample image, and the user instruction correspondsto the identification information input by the user.
 11. The imageforming apparatus according to claim 7, wherein the correction conditioncorresponds to a tone correction table for correcting a tonecharacteristic of the image data.
 12. The image forming apparatusaccording to claim 1, wherein the second sample image is arrangedbetween the first sample image and the fourth sample image in thepredetermined direction, and the third sample image is arranged betweenthe first sample image and the fourth sample image in the predetermineddirection.
 13. The image forming apparatus according to claim 7, whereinthe second sample image is arranged between the first sample image andthe fourth sample image in the predetermined direction, and the thirdsample image is arranged between the first sample image and the fourthsample image in the predetermined direction.